Combustion of coal



E. s. HAMMOND 2, 85,508

COMBUSTION OF COAL Filed Oct. 25, 1945 2 Sheets-Sheet 1 Sept. 25, 1945.

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P 1945- E. s. HAMMOND 2,385,508

COMBUSTION OF COAL Filed Oct. 23, 1943 2 Sheets-Sheet 2 INVENTOREdge/'57 Ham/non 0 Patented Sept. 25, 1945 fum'rso s'r 'rss PATENT has.

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E gar 8. Blow-Knox Jersey Hammond, smmnem, N. was

iOornpany, a corporation of New Appltcation oetoberzs, 194:, SerialNa'souu 2 cam (01. 241-) This invention relates to the combustion ofcoal and particularly to a method and apparatus for the combustion ofpulverized coal.

Systems utilizing pulverized coal. asheretofore known and used, are opento numerous objections such as high cost, limitations. upon the kind of.fuel usable therein, and non-uniformity and general difflculty'inoperation. My improved system overcomes these objections and provides aprocess and apparatus capable of satisfactorily burning .not only fuelwhich is unsuitable for firing in pulverized form by known equipment,

but even fuel of such character that it is usable only with diilicultyin any combustion system, for example, anthracite of considerablehardness .and substantial ash content.

A complete understanding of the invention .may be gained from thefollowing detailed description of a present preferred apparatus and theexplanation of its operation.

In the drawings: 1 Figure 1 is an elevation showing the wall of a boilerfurnace or the like in section, with a burner extending thereinto andcoal pulverizing apparatus connected to the burner;

1 Figure 2 is a section taken on the plane of line 11-11 of Figure 1with parts in elevation and parts omitted for the sake of clearness;

- Figure 3 is a central vertical section through the injector whichdelivers preliminarily ground coal to a final grinding zone or impactgrinder; and

Figure 4 is a central vertical section through the burner.

Referring in detail to the drawings, I have shown a burner l0,projecting into a port H in th wall l2 of a boiler furnace or othercom-- bustion chamber. A hopper l3 receives preliminarily ground coalfrom any suitable source, e. g., a conventional mechanical grinder. Thecoal delivered to the hopper I3 is preferably ground to such size thatit may readily be reduced to a fine powder by an impact grinder. The

other grinders, as. for example, that of Andrews Patent 2,032,827. Thegrinder i8 comprises a continuous ductclosed upon itself which mayconvenientlybe composed of lengths of pipe 2. and return bends 2| and 22secured together by coupling flanges 22. A steam-jet injector 24discharges tangentially into the bend 22 near one end thereof. Theinjector has a nozzle 25 discharging into a Venturi tube 28,.andconnected by a pipe 33 to a suitable source of superheated steam. Thenjector also has an-inlet branch 21.

A pipe 28 extends downwardly from the end of the feeder H to theinjector branch 21. The upper end of the pipe 28 projects above thefeeder and is provided with a valve 29 whereby to control the amount ofair entrained with the coal delivered by the feeder I 4 to the injector24 and thereby discharged into the impact grinder 9.

Th grinder I9 is alsoprovided with nozzles 30 spaced around the bend 22from the injector. These ,nozzles are supplied with superheated steamfrom a manifold 3| through connections 22. The grinder I9 is alsoprovided with an oil- Y the larger particles traveling to the outside.

Thus only the finest particles escape through the bleeder 24, the largerones continuing around their course for further impact grinding, and

hopper It delivers coal to a continuous feeder ll illustrated as of thehorizontal screw type. The hopper and feeder are mounted on a convenientsupport such as an elevated platform II. The

feeder II is driven by a motor it through a reducer I I and coupling I8, the motor and reducer being mounted on the platform as shown.

The preliminarily ground coal is delivered by the feeder I 4 to animpact grinder is which provides a final grinding zone. The grinder I!is preferably of the type shown in Kidwell et al. Patent 2,219,011(Figure 6) I may. however, use

eventual discharge through the bleeder. The expansion of the steam as itemerges from the nozzles 25 and 30 releases suflicient energy to causethe necessary circulation of the coal through the grinder and itspassage therefrom through bleeder 34 and suitable pipe connections tothe burner proper.

In use, the injector, impact grinder and associated piping will be wellinsulated against thermal loss so that there is no opportunity for theformation of free water vapor by condensation within the system. I haveomitted the insulation from the drawings in the interest of clear-.

ness. The expansion of the steam is substantially adiabatic, and theinitial degree of superheat of the steam is so regulated that after ex-'pansion it is still in superheated condition, 1. e.,

there is no saturated steam formed. Since the amount of entering air islimited, the cooling effect is slight and is also controllable. I

The coal withdrawn through the bleeder 34 consists of particles of suchfineness that they will remain suspended in the circulating gaseousmedium so long as it is moving and will settle therefrom only at a veryslow rate even when the gas is motionless. I utilize this fact to conveythe fuel to the burner by the energy resulting from the expansion of thesuperheated steam, and am thereby enabled to maintain the coal insuspension in the steam while it travels from the impact grinder tothburner. The bleeder pipe 34 serves to conduct the suspension of finelyground coal in steam from the grinder to the burner l through pipeconnections 35 and 35. The pipe 35 delivers the suspension to acentrifugal separator 31 the outlet of which is connected to the pipe36. The separator 31 is particularly important when utilizing the hardercoals and may be omitted, as explained hereinafter, in some cases. Theseparator operates in the known manner to remove a portion of thesuperheated steam from the suspension, allowing the coal and theremainder of the steam to proceed through the pipe 36 to the burner ill.The amount of steam removed by the separator may be controlled by anadjustable pressure-relief valve indicated generally at 38.

The burner II), as best shown in Figure 4, comprises an outer tub 39 andan inner tube 49. A partition 4| closes the annular space between theinner end of the tube 40 and the interior of the tube 39, and isprovided with holes spaced circumferentially thereof. The burner has anouter end wall 42 through which the pipe 36 extends so that thesuspension of coal in a stream of superheated steam is dischargedaxially through the tube 40. A pipe 43 having a control valve or damper44 therein extends from any convenient source of air under suitablepressure to the outer end of the tube 39. By this arrangement, air forcombustion is supplied through the holes in the partition 4| to theinterior of the tube 39 adjacent the inner end of the tube 40. Theresulting combustible mixture, when ignited, burns completely andefiiciently. The type of flame may be controlled by varying theproportions of air and fuel. The volume of air supplied may becontrolled by adjusting the damper 44 while the amount of fuel suppliedmay be controlled by varying the feeder l 4.

Practically all the air required for combustion of the fuel is suppliedto the burner through the pipe 43. A certain amount of air, it is'true,will inevitably be entrained with the coal entering the injector 24. Theamount of air so entrained will be controlled by the valve 29 so thatthe fuelair ratio in the grinder l9 and connections therefrom to theburner will be below the explosive value. This prevents th flame fromworking backthrough the pipe connections between the grinder and burner,even when the velocity of the fuel is below the rate of flamepropagation. The atmosphere in the grinder |9 is composed of superheatedsteam with a small amount of air, but the amount of air is insufiicientto make the suspension of coal combustible.

The method of my invention comprises: delivering preliminarily groundfuel to a final grinding zone, viz., that provided by th impact grinder[9; discharging the coal into this zone with a jet of superheated steamexpanding therein, to subject the coal to further grinding by impact inthe final grinding zone, and reduce it to pa ticles of such size thatthey remain suspended in th steam; conducting the suspension of coalparticles in the steam to the burner I 0 by the energy resulting fromexpansion, of the steam; the maintenance of the coal in suspension continuously between the final grinding zone and burner; the introductionat the burner of the amount of air required for eilicient combustion;and the limitation of the amount of air in the suspension between thefinal grinding zone and burner to an amount below that which wouldresult in a combustible mixture, or excessively cool the stream.

In the burning of the harder coals having a high ash content, for whichthe invention is particularly adapted, it may be necessary, in order togrind the coal to the desired degree of fineness. to utilize an amountof superheated steam greater than that which can be passed through thecombustion chamber without causing an objectionable loss of heattherein, since the steam which enters the combustion chamber, while notcondensed, will absorb a certain amount of heat therefrom as it israised to the temperature existing within the chamber. It is for thisreason that I provide the separator 31. By appropriately adjusting thevalve 38, I cause the separarator to remove from the system any steam inexcess of that required to maintain the flow of the suspended coalparticles to the burner. It will be understood, however, that theproportion of the steam removed by the separator 31 should not be greatenough to permit precipitation of the coal particles or otherwiseinterfere with their maintenance in suspension continuously between theimpactgrinder and the burner. If the coal being processed is relativelylow in hardness, less steam will be required for the impact grindingthereof and the total amount of steam employed for grinding may not begreater than that which can be passed through the combustion chamberwithout excessive loss of heat. In such case, the separator 31 isunnecessary and may be omitted or by-passed by suitable valves and pipeconnections (not shown). While the invention is particularly adapted forprocessing hard coal, it may be found advantageous to mix a smallpercentage of bituminous coal with the hard coal, in order to produce acombustible mixture which ignites readily when supplied to the burner.

I have found that particles of solid fuel which have been powdered bymutual attrition in a gaseous conveying medium under conditionsinhibiting the formation and presence of moisture are peculiarly welladapted for immediate combustion if conveyed to the combustion zone insuspension in the gaseous medium, and also that the combustion system ofmy invention has a convenience and flexibility in operation akin tosystems using fuel gas. This is probably due in part to the fact thatthe particles are given no opportunity to agglomerate and consequentlypresent a maximum surface area for combustion. This, however, is not acomplete explanation and the reasons for the remarkable qualities of thefuel are not fully apparent. The fact that the particles are reduced insize by mutual impact rather than by the attrition of grinding surfacesseems to impart a different quality to them. There is advantage, too, inhaving a limited amount of air present with the superheated.

steam during the fine grinding and a better quality of fuel results.

My invention constitutes an important improvement in coal combustion inthat it makes possible the eiiicient burning of hard coal having a highash content, which could not be burned satisfactorily either onmechanical stokers or in pulverized form when processed by equipmentpreviously available. The invention thus opens the industrial market tocertain types of hard coal which have not previously been salabletherein. The apparatus comprising the invention is simple and is largelyautomatic in operation so that the required personal attendance is keptat a The method and apparatus of my invention provide a flexibility ofcontrol both as to the firing rate and the Iuel-air ratio which make foreflicient operation under all load conditions.

Although I have illustrated and described but a preferred embodiment andpractice of the invention, changes in the structure and proceduredisclosed may be made without departing from the spirit of the inventionor the scope of the appended claims.

I claim:

1. In a method of preparing and supplying coal for combustion, the stepsincluding delivering preliminarily ground coal to a final grinding zone,discharging superheated steam into said zone so as to cause the coal totraverse a closed path repeatedLv, thereby further grinding the coal bymutual impact in said zone to reduce it to particles small enough to becapable of suspension in said steam, conveying the steam and the coalsuspended therein to a burner by the energy resulting from expansion ofthe steam, maintaining the coal in a state of suspension in the steamcontinuously between the point of introduction of the steam and theburner, maintaining the fuel-air ratio below the explosive value betweensaid point and the burner, and maintaining the steam in a superheatedcondition between said point and the burner.

2. In a method of preparing and supp yin coal for combustion, the stepsincluding delivering preliminarily ground coal to a final grinding zone,discharging superheated steam into said zone so as to cause the coalrepeatedly to traverse a closed path, thereby subjecting the coal tofurther grinding to reduce it to particles of a size capable ofsuspension in the steam, conducting the steam and the coal suspendedtherein to a burner under the pressure of the steam supplied to saidzone, maintaining the coal in a state of suspension in the steamcontinuously between the point of introduction of the steam and theburner, and maintaining the steam in a state of superheat up to theburner.

EDGAR S. HAMMOND.

